With progress of digital multimedia, digital communication technology and digital Audio or Video broadcasting technology, the digital television (DTV) market has exhibited strong development potential in many countries across the world. In contrast to conventional analog TV, DTV has many desirable advantages, like low power consumption, efficient use of spectrum, high quality of received programs, convenience in service integration and so on, and in particular, DTV can provide interactive TV services to enable more individualized services so that people may subscribe TV programs according to their favorites.
DTV provides such an individualized service through a payment system. To protect interests of the program providers and prevent unauthorized access, scrambling operation is performed on the video, audio, auxiliary data and other control data to be transmitted to each DTV device, so as to implement conditional access (CA). In this way, only authorized subscribers can de-scramble the video, audio and other data to view the desired program.
For the CA system itself, there are various standards currently provided with some basic schemes, but no uniform standard is defined yet. As a result, the internal mechanisms of various CA systems are often defined by each CA equipment vendor respectively. Accordingly, CA systems developed by various vendors are different in a sort of way and the encryption techniques are not compatible with each other. As such, a CA system selected by a network operator is generally private, that is, if a network operator deploys a system available from a CA equipment vendor, the subscribers can receive programs from the operator only when using the device products (such as DTV receivers (Set-Top Box) or DTV sets) matching with the CA system. If the operator changes its CA system, the DTV receiver of the subscriber will have to be changed accordingly. Therefore, in current DTV technologies, DTV receiving devices (DTV receivers or DTV sets) are usually bound with the CA system, which limits the development of DTV significantly. There is, therefore, a need for separation of the CA system from the DTV receiver, that is, separation of service cards from devices. By separating service cards from devices, device products may be independent of the services provided by the operator. This solution may release the restrictions upon the development of DTV industry that the CA system imposes, as well as provide a solid foundation for the extension of value-added services in the future.
To implement separation of service cards from devices, the first problem to be settled is to provide a standard interface for transmitting data streams between the CA modules and the DTV receiving devices. Data streams to be transmitted in DTV devices are generally transport streams (TS) that accord with MPEG (Motion Picture Experts Group) specification. Existing specific interfaces for transmitting MPEG TS data, such as SPI (Synchronous Parallel Interface) and ASI (Asynchronous Serial Interface), are not suitable to communicate the data having no TS frame structure, especially asynchronous data, such as control commands and only unidirectional transport may be conducted. Such specific interfaces are thus not suitable for separation of service cards from devices.
To implement separation of service cards from devices, interfaces which accord with PCMCIA (Personal Computer Memory Card International Association) specification, i.e. PC card interfaces, are employed in current European standard EN50221 CI (Common Interface) and American standard SCTE DVS295 HOST-POD (Point of Deployment), to transmit MPEG TS data. With regard to the interfaces for the two standards, signal definition related to the TS input and output is similar to that in SPI, i.e. two SPIs acting as the input and output. Furthermore, both Cl and HOST-POD have an 8-bit asynchronous I/O interface for transmitting control commands while transmitting MPEG TS, so as to implement intercommunication and interoperability between the two sides connected via the interface. However, when PCMCIA interface is adopted to implement bidirectional transport of MPEG TS and intercommunication, the interface is too complicated, the cost goes higher and the transfer rate is not high comparatively. Particularly, the two interfaces are dedicated for specific services and are not universal, thus cannot be applied to any other device for services other than the specific services.
With fast development of diversified services, subscribers always hope their purchased devices have good compatibility and extensibility. There is, therefore, a need for a more universal bidirectional data transport interface to implement separation of service cards from devices.
Among the current universal bidirectional data transport interfaces, USB (Universal Serial Bus) interface has excellent performance and is applied widely. USB interface is mainly characterized in following features: it can easily be extended to be compatible with different types of devices and has low cost; USB 2.0 supports the data transfer with high speed mode up to 480 Mbps; support real-time data (audio and video data) transportation, support for PnP (Plug and Play) function; and its physical interface is suitable for coupling directly to USB portable devices rather than through cables. Besides, USB interface further supports data transfer for the most fundamental types, such as control transfer for configuring ports in connection, and interrupt data transfer for transferring real-time and reliable data, like echo or response.
While USB interface has the above advantages, there are some pending problems for transmitting MPEG TS data by using USB interface directly, because specific interface definition and data transport protocol are used in USB. To implement bi-directional real-time transport of synchronous MPEG TS, for example, USB interface needs two logic channels as its input and output, respectively. Meanwhile, although each frame may transfer data of fixed bytes in USB, data is not conveyed at its practical fixed rate, but in bursts at the intrinsic rate of the shared bus, for example, 480 Mbps. If MPEG TS, which should be transmitted at a constant rate, is transferred via USB, a buffer is needed to smooth the data rate, which leads to difficulty in clock recovery or phase locking of the source data. Furthermore, USB interface adopts input/output transaction and table header, fixed-rate frame structure and synchronization and identifier field definition. Accordingly, the transfer efficiency will decrease due to mismatch in packet structure, when encapsulating data streams whose transportation requires a particular structure of synchronous MPEG TS packets.
To adapt to the increasing MPEG TS-related services, especially the development of DTV and the trend that future new digital devices are provided with universal data communication interface, there is a need for a bi-directional serial data transport interface, which has the functionality of a universal USB interface and can satisfy real-time application of MPEG TS.